An ultrasound diagnostic system has been extensively used in the medical field due to its non-invasive and non-destructive nature. The ultrasound diagnostic system can provide an ultrasound image of the inside of a target object in real-time without resorting to any incisions. However, the ultrasound diagnostic system suffers from inherent shortcomings of an ultrasound image such as a low signal-to-noise ratio and a limited field of view. Thus, an image registration between a CT image and an ultrasound image has been introduced to compensate for inherent deficiencies of the ultrasound image.
Generally, an image registration between a CT image and an ultrasound image is performed by means of a spatial sensor. Errors may occur during the image registration due to spatial movement variation in the inner part of a target object such as respiration. If an ultrasound probe is moved and a 2-D ultrasound image is newly obtained, then the spatial sensor is typically used to determine whether or not the newly obtained 2-D ultrasound image exists in a pre-obtained 3-D ultrasound image, and to extract a 2-D CT image corresponding to the newly obtained 2-D ultrasound image from a 3-D ultrasound-CT registered image.